docs: def _varLib_finder(source, directory="", ext="ttf"):
      """Finder function to be used with varLib.build to find master TTFs given
      the filename of the source UFO master as specified in the designspace.
      It replaces the UFO directory with the one specified in 'directory'
      argument, and replaces the file extension with 'ext'.
      """
      fname = os.path.splitext(os.path.basename(source))[0] + "." + ext
      return os.path.join(directory, fname)